The invention relates to a device for detecting properties of a moving web of paper, more specifically for production control in the paper making process, having an infrared lighting device operable to illuminate the paper web with infrared light and a detector device for detecting light reflected or transmitted by the paper web, the detector device having a light input portion.
A device as described above is disclosed in WO 98/40727, wherein the web of paper is illuminated by an infrared light source that is not described in any detail. A light exit portion for directing light onto the web and a light input portion for receiving light from the web are located on a holding member supported by a crossbar. The light input portion has a plurality of optical waveguides arranged side by side in the form of optical fibers of typically between 50 and 600 micrometers thick. The ends of the fibers are polished and oriented toward the paper web, and the fibers are fastened to the holding member and project therefrom toward the web.
The infrared lighting device is intended to sufficiently illuminate the region of the paper web onto which the light input portion is directed. Reflected or transmitted light can be measured thereby. The web of paper should be illuminated as uniformly as possible, and the light used should be as intensive and homogeneous as possible in the infrared range, more specifically in the so called near-infrared range, i.e., within the range above visible light ( greater than 800 nm) and more specifically within the range of 1.5 to 2.5 micrometers. Light in ranges of wavelengths outside of the range detected by the detector device is not required.
The prior art device allows for a high number of measuring dots arranged side by side on the web of paper. Unlike cradles traveling back and forth on crossbars, this device has the advantage that light detection is carried out in a stationary condition relative to the crossbar. Untested regions of the paper web are smaller than with a cradle. In principle, this is an advantage that preferably should be maintained.
The present invention aims at providing an infrared lighting device that is particularly suited for the measurement task in question, that is relatively inexpensive, and that provides precise illumination of the regions in which the measuring dots of the detector device are located in the simplest possible way. Adequate protection from dust and dirt should also be achieved. In the paper making process, considerable amounts of dirt, dust and so on are produced. The infrared lighting device should be configured in such a manner that it hardly becomes dirty and that it can be readily cleaned when dirty. As paper machines often operate without interruption for many months, cleaning should be possible while the paper machine is in operation.
Furthermore, it is the object of the invention to provide a lighting device that requires a relatively small number of parts and that allows easy assembly, reliable operation, and an intense, homogeneous illumination of those regions of the web of paper on which the measuring dots of the detector device are located.
Toward these ends, the present invention provides an infrared lighting device with a light source in the form of a long, rod-shaped infrared emitter contained within a housing, the housing having a front plate facing the web of paper. A tight-fitting exit window is arranged in the front plate through which the light from the light source passes to impinge upon the web of paper.
The long rod-shaped infrared emitter provides the advantage that one single illumination source suffices for a portion of the paper web to be tested that is at least one meter, preferably at least two meters wide. A considerable portion of the web of paper, which typically is many meters wide, can thus be detected by only one source of light. This has a considerable advantage over many individual light sources used for illumination. The number of component parts of the lighting device is considerably reduced, such that assembly, service, and maintenance are simplified. At the same time, the illumination of the web of paper is very good. It is easily possible to adjust the device to accommodate diverse distances between the light exit portion and the web of paper. Finally, the lighting device is easy to clean and hence can be readily maintained in a
Typical infrared emitters as they are utilized for the invention are sold as tubular infrared emitters in the trade name of ZKA for example by Heraeus Nobellight GmbH, Kleinostheim, Germany. These emitters are tubes made of quartz glass with at least one internal canal inside of which there is located an electrically heated filament. So-called twin tubular emitters with gold reflector have proved particularly suitable. Emitters of circular tube type with gold reflector are also suitable, though. The manufacturer mentioned also produces these infrared emitters as a halogen emitter and as a short-wave infrared emitter. According to manufacturer information, they are almost exclusively utilized in fields in which heat is needed, e.g., for the drying of paper and cardboard, the drying of printing colors, the drying of lacquers in enameling lines and so on. An application in the field of metrology has not yet become known. A desired one of the aforementioned ranges of wavelengths can be obtained by selecting an appropriate infrared emitter.
The rod-shaped light source is accommodated within a housing that is elongated in the same way as the infrared emitter. The housing is preferably sealed. As a result, dust and dirt are reliably prevented from entering the infrared emitter and its surroundings. Furthermore, the housing assures that the light from the infrared emitter exits only at places at which exit is wanted.
As noted, a tight-fitting exit window through which the light of the light source is incident onto the web of paper is arranged in the front plate of the housing. The window allows for a dust-tight configuration of the housing. The exit window is easy to clean from the outside. It remains clean on the inside, since the housing is closed.
In a preferred embodiment, the exit window is configured as a rod lens. The rod lens has substantially the length of the rod-shaped infrared emitter. By virtue of the rod-shaped infrared emitter in connection with the rod lens, the lighting device is easily made to span a portion of the paper web of at least one meter wide. Furthermore, only very few component parts are needed for the lighting device.
Preferably, the rod lens has the shape of a cylinder. The cylindrical shape has the advantage that it can be readily mounted in the front plate in a tight-fitting manner. Because of the circular cross-section, the rod lens need not be oriented in any particular manner, and the rod lens also resists high changes in temperature. When the materials of which the cylindrical rod lens and the front plate are made are thermally expanding, a relative movement between the lens and the front plate may take place. The portion of the exit window that projects downward can be cleaned with simple means.
In a preferred embodiment, the detector device has a plurality n of spherical lenses. The spherical lenses are tight-fittingly received in apertures in the front plate and are each optically coupled, within the housing, to an allocated optical fiber of the light input portion. The combination of the thus configured light input portion with the infrared lighting device described above proved very suitable for measurement tasks. An adjustment of the measuring dot in the region on the web of paper which is illuminated by the infrared lighting device can be achieved by simple means. The aperture of the optical fibers is corrected or the light is collimated respectively by means of the spherical lenses. The size of the measuring dot on the web of paper can thus be varied; the diameter for example can be varied between 5 mm and 10 mm. In this way, paper can be measured with a sufficient cross sectional resolution of the profile measurement. Depending on the distance from the web of paper, which typically amounts to between 20 and 50 mm, on the size of the measuring dot and on the cross sectional resolution, spheres made of sapphire or of other glasses, which are transmissive to spectroscopy and have a diameter typically ranging from 2 to 10 mm, are used. They are easier to manufacture than the customary lens-shaped convergent lenses and can be more readily fastened and sealed than the latter.
By virtue of the spherical lenses, the optical images on the paper web can be predetermined and controlled with much more precision. As the housing is sealed, the coupling region between the spherical lenses and the optical fibers is protected. The portions of the spherical lenses that are directed toward the web of paper can be easily cleaned. The high hardness of sapphire is advantageous in this regard.
The spherical lenses advantageously are replaceably arranged in the front plate so that spherical lenses of different diameters can be mounted into the front plate. Different images can thus be obtained depending on the purpose of the measurement. The same applies for the rod lens of the infrared lighting device.
By changing the angle formed between the optical axes of the infrared lighting device and of the light input portions, the spacing between the housing and the web of paper can be readily adjusted. The easiest way to achieve this is to adjust the angular position of the infrared emitter relative to the exit window.
It is particularly convenient to have the housing configured in a substantially elongated shape, more specifically to give it a length of at least one meter and preferably a length of at least two meters. The infrared emitter should have an equal length. With the usual widths of paper webs, only a few housings are needed, preferably arranged side by side in a staggered relation on a stationary crossbar, for illumination of the entire web of paper. To permit the exchange of individual housings during the paper making process, at least some of the housings preferably are connected to the crossbar by way of a longitudinal guide in which they are slidable relative to the crossbar. Individual housings can thus be removed from the crossbar without disturbing the paper making process.
Furthermore, it is advantageous to provide a cleaning device designed to clean those portions of the spherical lenses and their surroundings that are accessible from the outside on the front face and that are oriented toward the web of paper. It is thus made certain that dirt, which is inevitable in normal operation of the paper machine, does not restrict the optical conditions.
It is furthermore convenient to provide the outer side of the rod-shaped infrared emitter with a reflector layer, more specifically with a gold reflector layer, and to leave therein an exit region, or an exit slit, which opens toward the exit window. The infrared radiation is thus substantially focused or directed onto the exit window.